The present invention relates to an electronic component, in particular to an electronic component with Sn rich deposit layer on the part for electric connection, and the process for depositing the same.
Tin is known as a metal which has excellent corrosion resistance, and it can form an excellent bond with solder, so a surface with Sn deposit layer can be used to form good soldered connections, and the Sn deposit layer can provide excellent corrosion resistance for the surface of metal or metal alloy. As a result, Sn or Sn rich deposit layer is widely used in the manufacture of electrical equipments or electronic components (e.g. semiconductor elements). Generally, Sn or Sn rich material is deposited on the parts for electric connection (e.g., leads) of electronic components and substrate (e.g., electronic components mother board), so as to preserve good solderability and interconnection.
However, as well known, Sn is susceptible for whisker growth. This whisker growth is mainly caused by the growth of intermetallic compound (IMC), such as Cu6Sn5, in the grain boundaries of the Sn.
FIG. 1A is a photograph of the microstructure of a cut taken from a Sn deposit layer in the prior art. As shown in FIG. 1A, a Sn deposit layer 20 is deposited on a substrate 10. In the Sn deposit layer 20, an intermetallic compound 30 is formed in the boundaries between a Sn grain 40 and the substrate 10. FIG. 1B is a schematic view of the photograph as shown in FIG. 1A. The growth of the intermetallic compound 30 will bring a change of the volume and thus introduce stress in the Sn deposit layer 20. Besides, the irregular growth of the intermetallic compound 30 can introduce local stress in the Sn deposit layer 20. This kind of compressive stress in turn results in whisker growth. The presence and growth of whisker will adversely affect the electrics property of the electrical component, since if the whisker grows to sufficient length, it will bring a low impedance among the terminals of the electrical component, or even cause short in the electrical circuit and malfunction of the electrical component or electrical equipment. Especially, as the ever increasing density of the electrical components in the integrate circuit package, the distance among different terminals and the intervals of wirings in the substrate are getting more and more narrower, and the adverse influence of the whisker on the electrics property of the electrical component is getting more obvious.
To inhibit the presence or growth of whisker, another measure used in the prior art is to add a barrier layer, such as a nickel layer, between the substrate and the Sn deposit layer. However, this measure will complicate the manufacturing process of the electronic component and increase its cost. In another kind of technology used to prevent or inhibit whisker growth, the electronic component is subjected to a heat treatment. By this technology, the intermetallic compound in the boundaries between the Sn deposit layer and the substrate will grow as a sort of bulk diffusion at a higher temperature, so that the whisker growth is greatly inhibited. However, the process of this technology is also complicated, and the heat treatment sometimes may bring side effect on the electrical component. Furthermore, some electrical components are not suitable to carry out such a heat treatment.
Accordingly, there is a demand to provide a process for Sn depositing, which can prevent or at least retard whisker growth in a Sn deposit layer.